This invention relates to torque measuring devices and methods, and more particularly to noncontacting torque measurements useful for rotating machinery.
The Industrial Revolution introduced a vast amount of labor-saving machinery. This machinery, for the most part, involved the use of rotating members such as shafts. Even when reciprocating motions were used, as for example in fabric-weaving looms, the reciprocating motion was derived from the rotational motion of prime movers. Torque can be measured by the use of calibrated levers, which deform in an amount related to the torque applied between the shaft and the lever. However, there is no good way to measure the torque experienced by, or applied to, a rotating shaft. Certainly, a resistive strain gauge or strain sensor could be attached to the rotating shaft, and its resistance could be measured; the problem lies in coupling the electrical parameter to the underlying or non-rotating structure. Slip rings can be used, but introduce so much noise as to make the resulting measurement less useful than it might be. An electronic transmitter could be physically coupled to the shaft and electrically coupled to the strain gauge, but the transmitter needs to be designed to withstand the rotational forces, and a source of electrical energy must accompany the transmitter.
Improved torque sensing arrangements are desired.
A method is described for measuring the torque to which a torque-bearing member, such as a shaft, is subjected. The torque results in a strain along at least a portion of the length of the torque-bearing member or shaft. The method includes the step of affixing a piezoelectric transducer to the portion of the length of the torque-bearing member. The piezoelectric transducer electrically responds to mechanical strain by producing voltage, current, or charge (voltage); the transducer includes electrodes on which, or in which, the voltage is generated in response to strain. A signal responsive to the voltage is coupled to a signal-measuring instrument, for producing an indication of the magnitude of the signal, and consequently of the magnitude of the torque. In a particular mode of this method, the coupling step includes the step of coupling the voltage to the instrument by way of a capacitive device. A particularly advantageous mode of the invention includes the further step of rotating the torque-bearing member relative to an underlying structure or base. The step of coupling the voltage by way of a capacitive device comprises the step of coupling the voltage by way of (a) a generally annular first electrode coupled to an electrode of the piezoelectric transducer, where the first electrode rotates in consonance with the torque-bearing member, or (b) a generally annular second electrode concentric with the first circular electrode, which second electrode is spaced away from the first electrode by a gap, and is affixed to the underlying structure.
The electrical connections in the abovementioned method are made by the steps of connecting the piezoelectric transducer in circuit with an alternating electrical excitation arrangement so that an alternating voltage is applied across the piezoelectric transducer, upon the completion of which at least a component of the voltage appearing across the transducer is measured to determine the torque. The step of connecting the piezoelectric transducer in circuit may include the step of connecting the transducer in the feedback path of an electrical oscillator. As an alternative, the step of connecting the transducer in circuit may include the step of connecting the transducer to receive at least a portion of the output signal of an electrical oscillator-generator, also known simply as an xe2x80x9coscillatorxe2x80x9d. The affixing step may include the step of affixing to the torque-bearing member a holder carrying the piezoelectric transducer.
In another method according to an aspect of the invention, the torque to which a torque-bearing member is subjected is measured. The torque results in a strain along at least a portion of the length of the torque-bearing member. The method according to this aspect of the invention includes the step of affixing to the appropriate portion of the length of the torque-bearing member a piezoelectric transducer which changes its electrical properties in response to torque, and which includes electrodes for coupling signal between the transducer and other circuits. An alternating electrical signal is coupled through the transducer by way of the electrodes to produce an alternating electrical signal having at least one characteristic which depends upon the torque. At least the characteristic of the alternating electrical signal, as modified by the transducer, is measured, to determine at least one of the magnitude and phase of the torque. The step of affixing in this method may include the step of affixing to the portion of the torque-bearing member a holder which includes the piezoelectric transducer. In one of two alternative embodiments of this method, the step of coupling an alternating electrical signal includes the step of connecting the piezoelectric transducer in the feedback loop of an amplifier, to thereby form or define an oscillator which generates the alternating electrical signal with a frequency which varies in response to the characteristic of the piezoelectric transducer. In the other of the two alternative embodiments, the step of coupling an alternating electrical signal includes the step of connecting the piezoelectric transducer to receive the alternating electrical signal from an oscillator, which may be an external oscillator or oscillator-generator, which operates at a fixed frequency, as a result of which the amplitude of the signal component appearing across the piezoelectric transducer is responsive to the characteristic of the piezoelectric transducer. In this method, the step of measuring may include the further steps of (a) coupling the alternating electrical signal, which has at least one characteristic which depends upon the torque, to a first input port of a mixer, (b) coupling a sample of the alternating electrical signal from the oscillator to a second input port of the mixer, (c) in the mixer, mixing the alternating electrical signal, having at least one characteristic which depends upon the torque, with the sample of the alternating electrical signal from the oscillator, for thereby producing a baseband signal component, and (d) measuring a characteristic of the baseband signal component.
In a particular apparatus according to an aspect of the invention, a piezoelectric transducer arrangement includes a piezoelectric transducer including at least first and second electrical electrodes, and having defining dimensions. A base is included, which has an aperture dimensioned for accepting the piezoelectric transducer, and which further includes a fastening arrangement adapted for fastening the base to a torque-bearing structure. A holding arrangement is mechanically coupled to the base and to the piezoelectric transducer, for holding the transducer in the aperture. A first electrical connection arrangement is mounted on the base. A second electrical connection arrangement is also mounted on the base. A first electrical conductor is coupled to the first electrical electrode and to the first electrical connection arrangement, and a second electrical conductor is coupled to the second electrical electrode and to the second electrical connection arrangement. In a particular embodiment of this arrangement, a protective cover is mounted on the base over the piezoelectric transducer. In one embodiment, the arrangement of the base and transducer is monolithic. In some embodiments, the transducer includes more than two electrodes.
In a particular transducer arrangement, an included electrode structure provides the means for calibration and self-diagnosis protocol of the torque sensor. Specifically, application of external electrical signals by means of dedicated electrodes, andor application of optical signals, a torque or stress of known magnitude is created in the piezoelectric transducer, thereby allowing the sensor to be calibrated andor diagnosed in-situ.